Industrial filamentary mist eliminators are comprised of pads that are typically fabricated of layered knitted or woven metal or plastic filament mesh. Mist eliminators with filamentary structure may also be fabricated from non-woven fibrous media, such as porous air-laid mat bonded with resins. Mist eliminator beds of conventional tower packing elements, as well as beds of filamentary packing elements, such as those described in Lerner patents are also known to the art.
Knitted-mesh types of mist eliminators are typically woven from four to 11-mil filament diameters and have a capability of removing liquid droplets down to about 5 micrometers in droplet sizes. For finer drop removal, composite materials containing 10 to 50-micron diameter fiberglass or plastic fibers co-knitted with a heavier metal mesh framework are commonly used.
Conventional knitted-mesh mist eliminator pads typically constitute uniform porous media, which tend to retain liquid due to surface tension and counterflow gas-liquid frictional effects. Even at very low mist loadings, a liquid layer tends to builds up in the mesh at the bottom of the pad. This is particularly the case for the smaller mesh pore openings (denser mesh and finer filaments). The liquid layer typically builds to a level which provides enough gravity head to allow equilibrium drainage discharge from the mesh at the same rate mist accumulates within the pad. Frictional effects that retard liquid drainage arise from the fact that the upflowing gas is forced to rise through the same mesh pore channels that the captured liquid mist must use to back drain out of the pad. This competitive flow situation impedes liquid drainage.
Typically, pad thickness for effective mist removal is determined not by the requirement for filtering out the initial mist reaching the pad, but for filtering out the secondary mist generated in the pad by gas bubbling through the retained layer of liquid. This secondary mist is typically designated as re-entrainment. The limit on gas flow capacity of a mist eliminator is generally set by either the flood point or re-entrainment penetration point. For the purpose of this specification, the flood point is defined as that combination of gas and liquid rates at which liquid begins to rapidly accumulate within the pad with a correspondingly rapid rise in gas pressure drop across the mist eliminator. For the purpose of this specification, the re-entrainment penetration point is defined as that point at which spray generated by gas bubbling through the continuous liquid layer within the mist eliminator pad penetrates the upper surface of the pad.
Prior art for increasing the flood point and gas flow capacity of a mist eliminator pad or bed provides preferential liquid discharge paths from the pad.
Ozolins, et al., uses internally structured mesh pads in which there are zones of varied mesh density, while Lerner '593 provides external filamentary drainage rolls acting as appended liquid downspouts. Both means of facilitating liquid discharge from the lower portion of the mist eliminator pad or bed serve to decrease liquid retention in the pad and increase the gas re-entrainment velocity. These prior art mist eliminators thus typically operate at higher gas velocities than do conventional plain pad mist eliminators. However, in both Ozolins '806 and Lerner '593, liquid is discharged in the form of drops or streams directly into the approach gas flow, which is now at higher velocities than can be employed with a conventional pad.
It has now been discovered that the higher ranges of gas velocities achievable using the art of Ozolins or Lerner are high enough to entrain free-falling liquid drops. Additionally, the turbulent flow regime corresponding to the higher gas flow rates generates a maximum gas flow velocity in the central region of the containing vessel. Thus, while facilitated discharge of liquid from the pad serves to increase the gas velocity operating range, the increase in gas rate impedes the free fall of the liquid external to the pad, particularly in the central region of the vessel. Secondary failure of the pads of '593 and '806 has been found to occur by entrainment of the liquid discharge drops and stream external to the mist eliminator by the higher gas velocity approaching the pad. The new limiting flood parameter of pad or bed operation results from refluxing of the liquid back to the pad after it has been discharged from the pad or its appendages. Because the refluxing of liquid drops occurs outside of the pad, this secondary cause of pad flooding is a function of the approach gas velocity, not the internal pore gas velocity.
The prior art of augmenting internal liquid drainage in and from the pad, thereby increasing internal pad gas flood velocities, has thus created a new and undesirable secondary cause of flow limitation. That is, solutions to the problem of facilitating pad liquid release have led to an external problem of re-entrainment of the liquid discharge by the higher allowable gas velocities. A means of augmenting pad liquid drainage, without incurring the creation of the secondary flood mechanism limit external to the pad, is needed to achieve higher pad flow capacities and extend the operating range of filamentary mesh pads and beds.